In working draft N3126, subclause [re.regex.assign], paragraphs 12, 13 and 19, the name string_type is used. This is presumably a typedef for basic_string<value_type>, where value_type is the character type used by basic_regex. The basic_regex template however defines no such typedef, and neither does the <regex> header or the <initializer_list> header included by <regex>.